In continuous production of sheet glass, it is usual to use as a raw material feeding device a so-called blanket feeder which is mounted beside the glass melting tank. Powdered raw materials such as silica sand, soda ash, limestone, and cullet are mixed and then fed into the glass melting tank by the blanket feeder. During the downstream movement of the raw materials in the tank, the materials melt gradually by heat provided by the heated molten glass and two rows of burners arranged on both sides of the way along which the raw materials on the molten glass move. The molten glass which comes out from an outlet port of the tank is introduced into a suitable forming device to form rigid sheet glass. However, in this conventional method, there is a drawback which will become clear as the description proceeds.